So-called metal nanoparticles that are formed by pulverizing metals such as silver down to a nanosize level (i.e., 1 to a few hundred nanometers in size) have considerably large specific surface areas. Accordingly, they have been attracting attention as functional materials which can possibly be used in various fields of applications, such as catalysts, electronic materials, magnetic materials, optical materials, various sensors, color materials, and medical testing applications. However, when the size of a metal is reduced down to a nanosize level, the surface energy of metal nanoparticles increases which causes a lowering of melting point at the particle surfaces. As a result, the metal nanoparticles readily fuse with each other and the storage stability is deteriorated. Therefore, in order to stabilize metal nanoparticles, it is necessary to protect them with a protective material so as to prevent the fusion.
In general, solution methods, vapor phase methods and the like are available as methods for producing metal nanoparticles. However, as mentioned earlier, use of a protective material is essential regardless of the type of production methods employed, and various protective materials have been proposed to date. As a protective material, it is known that, for example, proteins such as gelatin and albumin and water soluble polymers such as polyvinyl alcohol and polyvinylpyrrolidone generally provide greater protection force than the surfactants of low molecular weight (for example, refer to Patent Document 1). However, because these water soluble polymers readily form aggregates with each other, the metal nanoparticles having these water soluble polymers as protective materials also form aggregates in many cases. In addition, because the protective materials generally bond poorly to the metal surface, metal nanoparticles cannot be protected stably. This could also be another disadvantage, and thus use of the protective materials may not be an ultimate solution for solving the problem of storage stability.
As a method for solving the above problems, the present inventors have already provided, by reducing metal ions such as silver nitrate in the presence of a terpolymer formed of a polyalkyleneimine chain with a hydrophilic polymer chain and hydrophobic polymer chain bonded thereto, a metal-fixed polymer associate in which metal nanoparticles are coordinated to, and thereby fixed to the polyalkyleneimine chain in the terpolymer (refer to Patent Document 2). According to the above method, an aqueous) dispersion formed of a terpolymer in which metal nanoparticles generated by the reduction of metal ions are retained can be obtained. However, counter ions of the metal ions used as a source material, for example, nitrate ions, remain dissolved in an aqueous medium, and it is necessary to conduct a process for removing the counter ions when using the obtained aqueous dispersion in an aqueous conductive ink, an aqueous coating material, or the like. It has been known that ionic components in the aqueous dispersion tend to remain within the surface of solid contents and cannot be removed with ease. Accordingly, it is necessary to employ a purification process such as dialysis which is usually not suited for industrial implementation. In addition, even when an operation such as centrifugation is conducted in order to recover only the metal-fixed polymer associates, an industrial implementation thereof still remains difficult, which leads not only to a complicated production process but also to an increase in cost for treating waste water produced in the production process. Moreover, implementation of such complicated post treatment step ultimately affects the association state of the metal-fixed polymer associates. As a result, it is possible that the storage stability of the metal-fixed polymer associates is impaired, and thus there is a need for further improvements.
Metal oxides such as silver oxide require no process for treating counter ions that are generated when reducing metal salts such as silver nitrate. Accordingly, various methods have been proposed in order to produce fine silver particles by using silver oxide as a source material, followed by the reduction thereof (for example, refer to Patent Documents 3 and 4). However, fine silver particles obtained in Patent Document 3 had an average particle size within a range from 1.5 μm to 2.5 μm, and Patent Document 4 only produced fine silver particles of 3 μm and thus had also failed to reduce the size smaller than the order of micrometers. When using them, for example, as a conductive material, so-called “nano-size effects” in which they can be fused at low temperatures cannot be attained, and a high temperature treatment of 400° C. or higher is required in order to achieve a practical level of conductivity. Accordingly, the types of substrates that can be used are limited, and it is also disadvantageous in terms of energy cost, and the like.
In addition, the silver-fixed polymer associates obtained in the aforementioned Patent Document 2 which used silver nitrate had a spherical shape or a polygonal shape, which had already been achieved by other conventional methods, and thus Patent Document 2 did not offer any method for controlling the morphology thereof. Nanostructures having a specific shape that is periodic as well as uniform, other than a spherical or polygonal shape, and which also have a particularly large surface are expected to be used in wide range of fields. Therefore, provision of such nanostructures using a simple and easy technique is of great industrial value.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. Hei 8-027307    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2006-213887    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. Hei 9-111317    [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2005-220380